Hydraulic lift

ABSTRACT

A hydraulic lift having an accumulator and an associated flow restricting valve disposed in a fluid conduit supplying a fluid under pressure for lifting a cage. In the hydraulic lift, the flow restriction of the flow restricting valve is varied in response to variations in the load carried by the cage so as to prevent transient oscillations tending to occur during starting, deceleration and stoppage of the cage.

United States Patent [1 1 Fujisawa et al.

[ 41 HYDRAULIC LIFT [75] Inventors: Fumio Fujisawa, Mito-shi; Mitsuaki Takenoshita; Ichiro Nakamura; Hiroshi Yumino, all of Katsuta-shi,

[73] Assignee: Hitachi, Ltd., Tokyo, Japan [22] Filed: Sept.l1, 1972 21] Appl. No. 287,833

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[52] U.S. Cl 60/413, 60/469, 187/17 [51] Int. Cl. FlSb 1/02 [58] Field Of Search...., 60/4l3, 414, 415, 60/907, 469; 187/17 [56] References Cited UNITED STATES PATENTS 3/l964 Kautz 60/415 X [451 Dec. 18, 1973 3,220,312 11/1965 Dal] ..60/469X Primary Examiner-Edgar W. Geoghegan Attorney-Paul M. Craig, Jr. et al.

[5 7] ABSTRACT A hydraulic lift having an accumulator and an associated flow restricting valve disposed in a fluid conduit supplying a fluid under pressure for lifting a cage In the hydraulic lift, the flow restriction of the flow restricting valve is varied in response to variations in the load carried by the cage so as to prevent transient oscillations tending to occur during starting, deceleration and stoppage of the cage.

5 Claims, 7 Drawing Figures PAIENTEDuEm 81973 SHEET 3 BF 5 a H m2; B

W m N w m Yin $73: $0 3 3 6. 5 H use. woo w m N m voo m mod X m QE w FIG. 7

AREA OF VALVE PORT (m LORD 1 HYDRAULIC LIFT This invention relates to hydraulic lifts and more particularly to a device for damping and preventing transient oscillations which tend to occur during starting, deceleration and stoppage of cages of the hydraulic lifts.

A device for damping oscillations of hydraulic lifts has been recently developed, according to which an accumulator having an orifice at the inlet portion thereof is connected to a conduit branched from a main highpressure fluid conduit. The function of the accumulator in the conduit system is such that it absorbs a portion of the fluid flow when the fluid pressure within the high-pressure fluid conduit tends to increase, while it supplies the fluid into the high-pressure fluid conduit to relieve an abrupt change of the fluid pressure in the high-pressure fluid conduit when the fluid pressure within the high-pressure fluid conduit tends to decrease. In other words, the accumulator acts to smooth out pressure variations within the high-pressure fluid conduit thereby preventing occurrence of undesirable ripples so that the cage can be accelerated and decelerated with a waveform substantially free from oscillations. Therefore, the desired smoothing of pressure variations cannot be attained if the fluid flows into and out of the accumulator at an excessively rapid rate or at a very slow rate. When the orifice has a low resistance to the fluid flow, the fluid flows into and out of the accumulator almost instantaneously, while when such resistance is quite high, a large, length of time is required for the fluid to flow into and out of the accumulator. In an extreme case in which the resistance is extremely high or low, the desired fluid pressure control cannot be attained and smooth operation cannot be expected. Thus, there is an optimum value for the resistance of the orifice disposed at the inlet portion of the accumulator.

On the other hand, the accumulator has preferably a greatest possible fluid absorbing and discharging capacity from the viewpoint of oscillation damping. However, the greater the capacity, the greater variations occur in the level of the cage at rest depending on the relative magnitude of the load carried by the cage, that is, the number of passengers in thecage. Thus, there is a certain limit in the accumulator capacity that can be practically employed.

It is an object of the present invention to provide a novel and improved device for efficiently damping transient oscillations tending to occur during starting, deceleration and stoppage of the cage of a hydraulic lift.

Another object of the present invention is to provide a device of the kind above described in which means is provided to control the rate of restriction on the flow of fluid into and out of an accumulator at an optimum setting depending on the load carried by the cage so as to prevent occurrence of transient oscillations.

In accordance with the present invention, there is provided a hydraulic lift comprising a hydraulic cylinder having a plunger, a cage arranged for upward and downward movement in response to the vertical movement of said plunger, a tank containing a hydraulic fluid therein, a conduit connecting said cylinder with said tank, a pump for pressurizing the hydraulic fluid drawn from said tank and supplying the fluid under pressure into said cylinder, an accumulator connected 2 to said conduit for absorbing ripples occurring due to pressure variations in the fluid under pressure, a variable flow restricting valve disposed in the path of the fluid under pressure into and out of said accumulator,

and means for varying the resistance of said variable flow restricting valve against fluid flow in such a manner that the flow resistance increases with the increase in the load carried by said cage.

Other objects, features and advantages of the present invention will be apparent from the following detailed description taken in conjunction with the accompanying drawing, in which:

FIG. 1 is a diagrammatic view of a hydraulic lift system for analyzing the oscillation characteristic of the hydraulic lift;

FIG. 2 is a graph showing variations in the ratio of damping the oscillations of the lift system relative to the flow resistance of an orifice disposed adjacent to r the inlet of an accumulator for the: purpose of oscillation damping;

FIG. 3 is a diagrammatic view showing an embodi' ment of the present invention;

FIGS. 4a and 4b are graphs showing how theacceleration varies relative to the load. carried by the cage when the flow resistance of a flow restricting valve disposed adjacentto the inlet of an accumulator shown in FIG. 3 is kept at a constant setting;

FIGS. 5a and 5b are graphs similar to those shown in FIGS. 4a and 4b, but showing the relation between the flow resistance and the load when the flow resistance of the throttle valve is varied to different settings;

FIG. 6 is a schematic view showing the structure of one form of the flow restrictingvalve shown in FIG. 3; and

FIG. 7 is a graph showing the relation between the area of the flow restricting portio n of the flow restricting valve and the load carried by the cage.

The time constant 1a of an accumulator system in a hydraulic lift must be kept constant in order that the accumulator system can exhibit the same condenser effect continuously without being affected by the load carried by the cage. The time constant 1a of the accumulator system is given by Ta Ca-Ra where Ra is the flow resistance of an orifice disposed adjacent to the inlet of the accumulator, and Ca is the capacitance of the accumulator. I

The capacitance Ca represents the condenser effect of the accumulator and is given by Ca Vai' Pailn'P where Pai is the pressure of a gas initially charged in the bladder of the accumulator, Vat is the volume of the gas at the gas pressure Pm and is the nominal capacity of the accumulator, Pao is the pressure of the gas in the bladder of the accumulator before actuation of the hydraulic lift and is equal to the pressure of the fluid in the hydraulic cylinder before the hydraulic lift is actuated, and n is the exponent inthe polytropic change. The gas pressure Pao varies depending on the load carried by the cage and a corresponding variation in the capacitance Ca results. In order to therefore maintain 1a constant, it is necessary to vary Ra depending on the variation in Pao. This will be described in detail with reference to the drawing.

F l0. 1 shows diagrammatically the flow of a hydraulic fluid in a hydraulic lift system. The basic idea for damping undesirable oscillations will be described with reference to FIG. 1. Referring to FIG. 1, a motor 1 drives a variable displacement hydraulic pump 2 which draws a fluid such as oil from a tank 3 through a lowpressure fluid conduit 4 and discharges the fluid under pressure into a hydraulic cylinder 8 by way of a highpressure fluid conduit 5. The fluid under pressure supplied into the hydraulic cylinder 8 forces a plunger 9 upward thereby causing upward movement of a cage 10 mounted at the top of the plunger 9. During downward movement of the cage 10, the hydraulic pump 2 draws the fluid from the hydraulic cylinder 8 to discharge the fluid into the tank 3. A drain conduit 15 having an orifice therein is provided for returning a pump leakage to the tank 3. An oscillation damping accumulator 18 is connected to the high-pressure fluid conduit 5 and a flow restricting orifice 19 is disposed in the path of the fluid into and out of the accumulator 18. The accumulator 18 is of the type in which a bladder 20 of rubber connected to a source of gas is disposed within a shell of steel.

How the damping ratio 4; of such a lift system varies relative to the diameter d of the orifice l9, hence the flow resistance Ra of the orifice 19, will now be calculated.

Continuity of flow is expressed by the equation ,,=A,\"+ V/BF'l-Qa where Q, is the theoretical quantity of the fluid discharged from the hydraulic pump, A is the pressure receiving area of the plunger, X is the displacement of the plunger, P is the fluid pressure in the high-pressure fluid conduit and hydraulic cylinder, Q, is the quantity of pump leakage, V is the volume of the fluid in the high-pressure fluid conduit and hydraulic cylinder, B is 'i the bulk modulus of the fluid, and Qa is the quantity of Q P Pa/Ra where R, is the flow resistance of the orifice in the drain conduit against the pump leakage, Pa is the pres sure of the gas in the bladder, and Ra is the flow resistance of the orifice disposed adjacent to the inlet of the accumulator.

Suppose that the gas pressure is equal to the fluid pressure in the accumulator, then Qa is expressed as Qa Ca'Pa 5 where Ca is given by LII and (7) arederived from the equation relating to the state of gas Pa Va" constant where Pa is the pressure of the gas, Va is the volume of the gas, and n is the exponent of the polytropic change. From the condition of the isothermal change given by Pai' Vai Pao' Vao the capacitance Ca of the accumulator is given by Ca=Vai'Pai/n'P,

where Pai is the initial pressure of the gas charged into the bladder of the accumulator and Vai is the initial volume of the gas at the gas pressure Pai. From the equations (5) and (6), the following equation is obtained:

'ra'Pa Pa P where 'm is given by Ta Ca'Ra The equation l l indicates the fact that the fluid pressure within the accumulator varies with a first order lag relative to a variation in the fluid pressure within the high-pressure fluid conduit.

The motion of the cage-plunger assembly is given by m= AP where m is the mass of the cage and plunger. The transfer function E(S) representing the correlation between the quantity Qp of the fluid discharged from the pump and the acceleration X of the cage can be obtained by the Laplace transformation of the equations (3) to l3) and necessary calculation thereon as follows:

Therefore, the Characteristic equation of the lift system 1/1,, (1 +1 wh (1/,,T,, w )S (D /T, O (IS) The flow resistance Ra of the orifice disposed adjacent to the inlet of the accumulator is given by where p. is the coefficient of viscosity of the fluid, d is the diameter of the orifice, and l is the length of the orifice. The equation may be expressed as and l in the equation l 7) represents the damping ratio of the lift system. The damping ratio r is preferably as great as possible from the viewpoint of oscillation damping. W I

FIG. 2 shows the results of calculation of the damping ratio t relative to the flow resistance Ra and diameter d of the orificedisposed adjacent to the inlet of the accumulator. The relation between Ra or d and g differs depending on the loaded condition of the cage. In FIG. 2, the solid curve, one-dot chain curve and dotted curve represent a 0 percent load condition, a 50 percent loaded condition and a .100 percent loaded condition respectively of the cage. In each curve, there is a value of Ra or d which gives a maximum value of the damping ratio The flow resistance Ra giving the maximum damping ratio 5 increase with the increase in the load. Thus, the maximum damping ratio can be always obtained by. increasing Ra with the increase in the load.

The present invention is based on the finding above described and contemplates to vary the flow resistance Ra depending on the load. Thus, when the flow resistance Ra is varied to various values R R and R, as shown in FIG. 2 depending on the 0 percent, percent and I00 percent loaded conditions respectively, the accumulator can operate to give always the maximum damping ratio Q.

An embodiment of the present invention will now be described in detail with reference to FIG. 3. Referring to FIG. 3, a motor 2] drives a variable displacement hydraulic pump 22 which draws a fluid such as oil from a tank 23 through a low-pressure fluid conduit 24 and discharges the fluid under pressure into a hydraulic cylinder 28 through a high-pressure fluid conduit 25, a pilot type check valve 26 and a high-pressure fluid conduit 27. The fluid under pressure supplied into the hydraulic cylinder 23 forcesa plunger 29 upward thereby causing upward movement of a cage 30 mounted at the top of the plunger 29. In'the case of downward movement of the cage 30, the hydraulic pump 22 draws the fluid from the hydraulic cylinder 28 to discharge the fluid into the tank 23. During upward movement of the cage 30, the pilot type check valve 26 is biased to the open position when the fluid pressure in the highpressure fluid conduit 25 overcomes the fluid pressure in the high-pressure fluid conduit 27. Duringdownward movement of the cage 30, the solenoid 32 of the solenoid operated change-over valve 31 is energized for supplying the fluid under pressure to the check valve 26 by way of pilot conduits 33 and 34 thereby forcedly biasing the check valve 26 to the open position. A drain pipe 35 is provided for draining the fluid into the tank 23 from the hydraulic pump 22, and a control motor 36 is connected to the hydraulic pump 22 by a shaft 37 for varying the capacity, hence the quantity of the fluid discharged from the pump 22.

An accumulator 38 is connected to the high-pressure fluid conduit 27 through a variable flow restricting.

hydraulic cylinder 28 is supplied by a conduit 41 into I a pressure chamber 53 of the variable flow restricting.

valve 39 to apply pressure to a spool 51 which is normally biased by a spring 50 toward the port connected to the conduit 41. Another port 54 is connected to the highpressure fluid conduit27 by a conduit 52 for the supply and discharge of the fluid under pressure to and from the accumulator 38. Thus, the area of the port 54 is decreased with the increase in the fluid pressure ap plied through the conduit 41 to provide an increased flow resistance corresponding to an increase in the load carried by the cage 30.-In other words, the fluid pressure in the high-pressure fluid conduit 27 increases with the increase in the number of passengers in the cage 30, and the valve 39 responds to this fluid pressure so that the area of the port 54 is decreased in a manner as shown in FIG. 7 thereby decreasing the quantity of the fluid supplied to the accumulator 38 through this port 54. Therefore, the maximum damping ratio C can be obtained always irrespective of the load carried by the cage 30 as described with reference to FIG. 2.

FIGS. 4a and 4b are graphs showing the acceleration of the cage relative to time when Ra in FIG. 2 is set at Ra R constant under a 0 percent loaded condition and a loaded condition respectively of the cage. The curves show the actually measured values of the acceleration of the cage when the cage is moved 'upward from the starting point to the stopping point. In FIG. 4a showing the acceleration under the 0 percent loaded condition, the flow resistance is such that the damping ratio r takes its maximum valve. Thus, the first wave alone of the transient oscillations appears in FIG. 4a and a good waveform can be obtained. However, in FIG. 4b showing the acceleration under the 100 percent loaded condition, the damping ratio I, takes the value represented by the point a in FIG. 2 instead of the maximum value, resulting in unsatisfactory damping of transient oscillations. I

FIGS. 50 and 5b are graphs similar to the graphs shown in FIGS. 4a and 41!; respectively, but differ from the latter in that Ra is automatically variable depending on the load carried by the cage by virtue of the provision of the variable flow restricting valve 39 as shown in FIG. 3. More precisely, Ra is automatically varied to R and R in FIG. 2 depending on the 0 percent and I00 percent loaded conditions respectively of the cage. Thus, the first wave alone of the transient oscillations appears in both the conditions and a good waveform can be obtained in either loaded condition.

lt will be understood from the foregoing description that the present invention provides a hydraulic lift giving a comfortable sense of ride due to the fact that the flow of a hydraulic fluid through the flows restricting valve is varied to suit the load carried by the cage so as to maintain the damping ratio at or in the vicinity of the maximum value. Further, although a complex speed control has been employed in prior art hydraulic lifts so as to minimize occurrence of ripples in the pressure of hydraulic fluid, the speed control can be very simply attained according to the present invention due to the fact that the damping ratio can be maintained always in the vicinity of the maximum value.

While the present invention has been described with reference to an embodiment in which fluid pressure is detected for indirectly detecting the load carried by the cage thereby varying the flow resistance, it is apparent that the same effect can be attained by detecting the load within the cage by any suitable known means and controlling the variable flow restricting valve depending on the load thus detected.

What is claimed is:

1. A hydraulic lift comprising a hydraulic cylinder having a plunger, a cage arranged for upward and downward movement in response to the vertical movement of said plunger, a-tank containing a hydraulic fluid therein, a conduit connecting said cylinder with said tank, a pump for pressurizing the hydraulic fluid drawn from said tank and supplying the fluid under pressure into said cylinder, an accumulator connected to said conduit for absorbing ripples occurring due to pressure variations in the fluid under pressure, a variable flow restricting valve disposed in the path of the fluid under pressure into and out of said accumulator, and means for varying the resistance of said variable flow restricting valve against fluid flow in such a manner that the flow resistance increases with the increase in the load carried by the cage.

2. A hydraulic lift comprising a hydraulic cylinder having a plunger, a cage arranged for upward and downward movement in response to the vertical movement of said plunger, a tank containing a hydraulic fluid therein, a conduit connecting said cylinder with said tank, a pump for pressurizing the hydraulic fluid drawn from said tank and supplying the fluid under pressure into said cylinder, an accumulator connected to said conduit for absorbing ripples occurring due to pressure variations in the fluid under pressure, a variable flow restricting valve disposed in the path of the fluid under pressure into and out of said accumulator, and means for varying the flow restriction of said variable flow restricting valve in such a manner that the flow restriction increases with the increase in the load carried by said cage.

3. A hydraulic lift as claimed in claim 1, in which said variable flow restricting valve is of the fluid pressure actuated type and responds to variations in the fluid pressure within a high-pressure conduit portion of said conduit due to variations in the load carried by said cage so that the flow restriction increases with the increase in the load carried by said cage.

4. A hydraulic lift as claimed in claim 1, in which the flow resistance of said variable flow restricting valve is varied in response to variations in the load carried by said cage so as to maintain the damping ratio at or in the vicinity of the maximum value.

5. A hydraulic lift as claimed in claim I, in which said accumulator is connected to a high-pressure conduit portion of said conduit between said hydraulic cylinder and a pilot type check valve, and said variable flow restricting valve is disposed in the high-pressure conduit portion between said accumulator and said hydraulic cylinder. 

1. A hydraulic lift comprising a hydraulic cylinder having a plunger, a cage arranged for upward and downward movement in response to the vertical movement of said plunger, a tank containing a hydraulic fluid therein, a conduit connecting said cylinder with said tank, a pump for pressurizing the hydraulic fluid drawn from said tank and supplying the fluid under pressure into said cylinder, an accumulator connected to said conduit for absorbing ripples occurring due to pressure variations in the fluid under pressure, a variable flow restricting valve disposed in the path of the fluid under pressure into and out of said accumulator, and means for varying the resistance of said variable flow restricting valve against fluid flow in such a manner that the flow resistance increases with the increase in the load carried by the cage.
 2. A hydraulic lift comprising a hydraulic cylinder having a plunger, a cage arranged for upward and downward movement in response to the vertical movement of said plunger, a tank containing a hydraulic fluid therein, a conduit connecting said cylinder with said tank, a pump for pressurizing the hydraulic fluid drawn from said tank and supplying the fluid under pressure into said cylinder, an accumulator connected to said conduit for absorbing ripples occurring due to pressure variations in the fluid under pressure, a variable flow restricting valve disposed in the path of the fluid under pressure into and out of said accumulator, and means for varying the flow restriction of said variable flow restricting valve in such a manner that the flow restriction increases with the increase in the load carried by said cage.
 3. A hydraulic lift as claimed in claim 1, in which said variable flow restricting valve is of the fluid pressure actuated type and responds to variations in the fluid pressure within a high-pressure conduit portion of said conduit due to variations in the load carried by said cage so that the flow restriction increases with the increase in the load carried by said cage.
 4. A hydraulic lift as claimed in claim 1, in which the flow resistance of said variable flow restricting valve is varied in response to variations in the load carried by said cage so as to maintain the damping ratio at or in the vicinity of the maximum value.
 5. A hydraulic lift as claimed in claim 1, in which said accumulator is connected to a high-pressure conduit portion of said conduit between said hydraulic cylinder and a pilot type check valve, and said variable flow restricting valve is disposed in the high-pressure conduit portion between said accumulator and said hydraulic cylinder. 